Compound positive displacement pump circuit



Oct. 22, 1940. H. F, vlcKERs COMPOUND POSITIVE DISPLACEMENT PUMP CIRCUIT Original Filed May l, 1937 2 Sheets-Sheet l N YENTOR. dry j? cZer A TTORNEY H. F. VICKERS COMPOUND POSITIVE DISPLACEMENT PUMP CIRCUIT Original Filed May 1, 19257 2 Sheets-Sheet 2 o y, fr w m IIMN lll/lf /l s Patented os`194c PATENT oFFlcE comouNn POSITIVE DISPLACEMENT 'PUMP cmcUrr Harry `F. Vickers, Detroit, Mich., assignor to Vickers, Incorporated, Detroit, Mich., a corporation oi Michigan Applicaties my 1, 1937, sei-15.1 No. 140,074

. Renewed August 28, 1939 i o 18 Claims. This invention relates to a compound positive displacement pump circuit. i It has particularly to do with a pump circuit having two or more positive displacement pumps in series to produce high pressures.L

There` have been numerous attempts to `con` nect two or more positive displacement pumps in series to increase `operating pressure but they haveproved unsuccessful because no means or method could be devised which would prevent one pump taking the whole pressurev J 'I'he object ofthe present inventicn'is to'provide a variable pressure `pumping system for positive displacement pumps which will allow two or more of suchpumps to be connected in serieswhen high pressure exists in the systeml said pumps each bearing a proportlonateshare of the, load. A further object is to provide apumping system which will allowrtwo icrmore pumps to operate at their highest efilciencypoints to provide a pressure greater than either is "capable oi emciently obtaining. o n

A still further object is toprovide a compound pump system having a relief valve whichnormally handles'the combined high pressure of both pumps, when operating in series, but which will also yrelieve the pressure output line at the `proper relief pressure of a single pump it one of the pumps fails to operatein 'series for one reason or another,

Briefly, the invention consists of a pump'circuit which includes two or more positive displacement pumps, suitable valves and conduits `which permit (assuming two pumps) both pumps to deo liver the sum of their two volumes into a common y line or conduit until the pressurereaches a certainpredetermined point, at which point a valve is automatically operated to cause one pump to deliver itsvolume into the intake of the other pump, both pumps thereafter working in series to deliver thesum oi the pressures Aoi the two pumps with the volume oi the last pump in the series. 'Ihe increase in pressure inthe system can be handled with substantially the same horsepower as is necessary for low Ipressure delivery. Illustrations oi the invention utilizing two and three pumps are disclosed in the accompanying drawings and the following description. The pumps are saidtobe in series when the out- V0 put of one is directed into thehinlet oi another and so on, the first and lastin the series being respectively connected to the liquid supplyI source and the main pressureline. i i

In thedrawings: I Fig. 1 shows the relationship oi the tank,

pumps, and motor of the compound pumping circuit, and shows the valves in said circuit in cross-section.

2 illustrates the circuit with three Dumps arranged `to operate in parallel and in series. 5

With reference to Fig. 1 of the drawings, a mo- Vtor I drives positive displacement pumps `2 and 3, said pumps being connected to a tank `I nlled withoil or othersuitable liquid byV conduits 5 and-8, respectively. Pump `2 a lower stage pump 10 is designed to have a slightly larger volume output or capacity than pump 3. An outlet conduit 1 from anupper stage pump 3 leads to a pressure utilizing `means (not shown), as for instance a vmachine tool, through a relief valve 8 and a con- 15 duit 8. The operating principles oi.' the relief valve 8 are explained in my Patent #2,043,453. Conduit 9a serves as a return from the pressure l utilizing means to the tank.

The outlet of pump 2 is connected to the con- 20 duit` 1 through a conduit Ill. a bypass valve II, an elbow I2, a check valveV I3, and a pressure conduit I4.' Pressure conduit Il and the housing of `the bypass valve II are also connected by a conduit IB which leads, through passageways lia 25 and Iib, to opposite sides of a piston I8 in the valve I I, said piston being backed lby a light spring Ia. It will be seen that, since both sides of the piston I8 are exposed to the main pressure line 1, the effective working pressure area is equal 3o to the difference in working pressure areas of the two sides. This permits the use of the relatively light spring ISG.

Between an inlet conduit I1 of pump 3 and the relief valve- II is a conduit I8, which is open to 35 the pressure outlet I0 of pump 2 when the piston valve IB opens a port I9. The elbow I2 in the pressure outlet of pump 2 is connected to the reliei valve 8 by a conduit 20. Ihe relief valve 8 consists of valve housing 2l which contains a 40 valve seat 2-2 and a piston valve 23 which is formed integrally .with a piston 23a. One side "of the piston 23a is directly exposed to the pressure outlet of pumps2 and 3 in conduit 1 while the other side is open to the liquid pressure in 'said .f5 conduit 1 through a small passageway 24 in the valve housing 2 I.` `The chamber behind the valve piston 23a contains alight spring 25 which tends to close the valve 23, and said chamber is also open to the conduit 20 except for a spring oper- 50 ated valve 28.

A valve assembly 21 with a slidable piston 80 is located near the entrance to the pump 8 and is open on one side through a conduit 28 tothe pressure outlet conduit 1 and on the other side 55 the main lines to conduit I8 and pump inlet I1. Between the tank conduit 6 a n d the valve 21, is a bypass conduit 29 which is open to conduits I1 and I8 when the slidable piston 30 is moved to the left from its position in the drawings. A check valve 3l is located in the line 8 between the tank 4 and the inlet I1.

The liquid in the system is preferably oil, and Will hereafter be referred to as oil. By design, pump 2 has a slightly larger capacity than pump 3,

In the operation: Assuming for purposes of illustration that a high pressure of 1500 pounds is required by a. machine tool in the pump circuit, thebypass ./alve II will be set for 750 pounds. The pumps 2 and 3 each have a maximum capacity of around 1500 pounds though neither is expected to carry that load. When the motor I is started the pumps 2 and 3 Will deliver a large volume of oil into the pressure line 9, the pump 3 discharging through conduit 1, and the pump 2 discharging past the bypass valve II and the check valve I3 through the conduit I4 to conduit 1.

If for any reason there occurs a restriction in the pressure line 9 or if the machine tool requires a higher pressure so that the oil pressure in the system rises to '150 pounds, the bypass valve I6 will be moved. to the left against the spring I6a due to the differential action vof pressure on the piston I6. This will -open the port I9 and bypass the discharge of the pump 2 to conduit I8 and in turn to the inlet I1 of the pump 3. Pump 3 was previously pulling oil .from tank 4, the valve 30 being closed, However, after the opening of the valve II the pump 3 will receive its supply from the conduit I8 and, since the'pump 2 has a slightly larger capacity than the pump 3, the check valve 3| will'be closed and the valve 30 may open slightly to allow the surplus oil to return through con-- duits 29 and 6 to the tank. The valve piston 30 is so constructed that the working pressure area exposed to the conduit 28 has a definite relationship to the working pressure area exposed to the inlet of pump 3. Assuming, for example, that this relationship is one to two, thepressure beu hind the pump 3 will be one-half the pressure in I and 9. With this relationship it will be seen that the pump 2 will be bearing a. definite proportion of the main pressure. For instance, if the pressure in the main line is 1500 pounds the load on the pump 3 will be 750 pounds, and the load on the pump 2 will also be 750 pounds.

As long as the pressure in the main line is over 750 pounds, which is the opening point of the valve I I, pump 2 will bear one-half of the pressure. If the pressure drops to 750 pounds, then the pumps will cease to operate in series and pump 2 will discharge directly into the main line. The check valve I3 prevents the backing up of the main line pressure after the valve II has opened.

The discharge capacity'of the pumps when operating in series will equal the capacity of the last pump in the series.

The relief valve 8 is a bypass valve. The valve is set for 750 pounds, but under normal conditions, when the pumps are working in series at 1500 pounds, the spring operated valve 26 is backed by 750 pounds liquid pressure in conduit 20, so that the valve 8 really has the effect of 1500 pounds. If, however, the pumps failed to work in series, then the pressure on the upper stage pump 3 might rise to over 1500 pounds and cause a failure of the pump. Under these conditions, however, if the lower stage pump 2 has failed, the valve 26 will no longer be backed by .the pressure therefrom and the valve 8 Will open.

combination relief andA at 750 pounds to allow the oli to bypass through port 22 to the tank. If pump 3 alone fails, discharge of pump 2 will be through valve 30 to the tank.

As previously indicated, more than two pumps can be introduced into this circuit in such manner that each will bear a proportionate share of the line pressure after a certainl point. It is preferable, of course, that each pump in the series have a slightly larger capacity than the one next adjacent it on the Work side of the series so that the last pump in the series opening into the pressure lines has the smallest volume capacity. This compound circuit allows all the pumps to operate at a pressure within their highest efflciency ranges, even though the maximum pressure in the system is such that none of the pumps could economically carry it alone. 'I'he term positive displacement pump is used in the specication to designate what is generally known as a constant displacement pump, this type of pump being most suitable for the circuits herein described.

Referring to Fig. 2, a circuit is shown with three positive displacement pumps which are also adapted to operate in parallel up to a certain pressure and to operate in series for pressures above a certain minimum. Like reference numerals indicate like parts in Figs. 1 and 2. In Fig. 2, an additional pump is indicated as.32 so that when the pumps are in series the low stage pumps will be 32 and 2 and the vupper stage pump will be 3. A differential proportioning valve 21a is provided for pump 2 while the proportioning valve 21 operates in con-junction with pump 3; The bypass valve II serves as before to direct pressure from the pump 2 to the inlet of pump 3 and a similar bypass valve I Ia serves to direct the outlet pressure from pump 32 to the inlet of pump 2. 'I'he relief valve 8 is the same as that shown in Fig. 1 except that the control element 26 is now adapted to be backed by the combined pressures of pumps 32 and 2 when such are operating in series. An additional relief valve 8a serves specifically for lpump 2.

assuming for purposes of illustration that a pressure of 1500 pounds is required by a machine tool in the pump circuit, each of `the bypass valves II and IIa will be set for 500 pounds. Pumps 32, 2 and 3, each have a capacity of around 1500 pounds but no single pump is expected to carry that load when the most efficient load for each pump is around 500 pounds. The pumps 32, 2 and 3 have different volume capacities, these capacities being slightly decreasing in the order named. When the pump motors I, Ia and Ib are started, the pumps 32, 2 and 3 acting in parallel will deliver a large volume of oil into the pressure in the system rises above 500 pounds, the bypass valves IIa and II will be opened in the order named s0 that the discharge of pump 32 will be directed to the inlet of pump 2 through conduit I 8b and the discharge of pump 2 will be directed to the inlet of pump 3 through conduit I 8a. The differential areas of Valve pistons 30 and 30a and valves 21 and 21a are so proportioned that each of the pumps will bear one-third of the total pressure in the line 9; For example, assuming that the pressurein line 9 is 1500 pounds, the pressure at the discharge of pump 2 and the inlet of pump 3 will be 1000 pounds, while the pressure at the 75 discharge of pump 32 and the inlet of the pump 2 will be 500 pounds. y

The relief valve 3 is a combination relief and bypass `valve as described ln connection with Fig. 1, and is normally set for the pressure of pump 3which, in the example above given, would be 500 pounds. However, when the pumps are operating in series, the pressure from the discharge ofpump 2 will be transmitted from the conduit 13a through conduit 20a to the control element 23 in the relief valve 3 so that the actual effect of the relief valve 0 will be 1500 pounds. Relief valve 8a is similarly arranged so that the control element 26a is backed by 500 pounds through the conduit 20h which is exposed during the series operation tothe pump 32.` The effective pressure of this valve is 1000 pounds. In practice, the spring 25 in relief valves 3 and 8a would be set for slightly higher than 500`poundsto insure opening of the valves Il and Ila prior to the opening of the relief valves. Likewise the spring in valve Il would be set slightly higher than the spring in valve I la to insure the pump 32 cutting into the series'operation before the pump2.

If pump 32 alone should fail, the pump 2 would draw liquid directly from the tank. Since there would be no pressure in conduits Ib and 20h, the effective pressure of relief valve 3a would be 500 pounds; consequently, the load on pump 2 could not exceed that amount. However, the bypass valve Il would operate as above so that at least 500 pounds pressure could be exerted through the conduit 28 to the control element 26 of the relief valve 3. Effective pressure of the relief valve AI I would then be 1000 pounds and the load on thepump 3- cannot exceed 500 pounds.

If pump 2 alone should fail, pressure on both sides of the 4piston 30a would be equal and, due to the larger area `on the right end, piston 30a would move to the left and allow the delivery of pump 32 to return tothe tank through conduit 23a. Pump 3 in this case would continue to operate but since pressure could not exist in line 20a, relief valve 8 would open at 500 pounds. If pump 3 alone should fail, the effect would be had on the piston 30 andthe entire discharge of pumps 32 and 2 would pass tothe tank through the conduit` 29. A o

If lower stage pumps I and 2 should fail, no pressure could exist in conduit 20a and consequently, the relief valve B would be effective at 500 pounds to-,prevent overloading of pump 3. Failure of pumps 2 and 3 would permit pressure over 500 pounds from pump 32 to pass to both sides of the piston 33a through theconduit i3b. This wouldv move the piston 30a to the left and bypass the discharge of pump 32. Similarly, in case of failureof pumps 32 and 3, the discharge `of the `remaining pump 2 would bypass through line 23 to the tank.

WhatI claim is:

l1. In a compound pump circuit, a plurality of positive displacement pumps normally connected to a main pressure line, pressure controlled valve means operative to connect said pumps in series at a certain predetermined pressure in the main line, valve meansresponsive tothe inlet and outlet pressures of each of said pumps butithe first in series to predetermine the relative proportions of pressure to be carried vby each pump when operating in series, and relief valve means connected to said main pressure line and normallylset for the pressure of the last pump in series and provided with `a control element backed up by the pressure oi.' the remaining pumps in series when ysuch are operating.

2. In a compound positive displacement pump circuit having a liquid supply source, a plurality of positive displacement pumps normally connected to a main pressure line, and adapted to be connected in series, conduit means for connecting the outlet of each pump but the last in series to the inlet of the next succeeding pump, pressure controlled valve means in said conduit means operative at a certain predetermined line pressure to open said conduits and connect the outlet of each of said pumps to the inlet of the next succeeding pump, thereby placing the liquid supply source in series with` the main pressure line through each of said pumps in turn, differential valve means at the inlet of each of said pumps but the first in series to regulate the relative proportions of the total pressure to be said pumps operate in series, and adapted t0` open at a proportionately lower predetermined pressure when any of said pumps fail to operate in series. v

3. In a compound positive displacement pump circuit adapted to produce a total circuit pressure greater than the pressure load on any one pump in the circuit when said pumps are connected in series, upperand lower stage pumps, means for regulating the relative proportions of the circuit pressure to be carried byupper and lower stage pumps when said pumps are connected in series, said means comprising a casing and a differential pressure area piston valve open on one side to the circuit pressure and on the other side to the discharge pressure of the lowerk stage pump, the respective pressure areas of said piston valve having a ratio substantially equal to the ratio to be maintained between the lower` stage pump pressure and the circuit pressure, said casing having a bypass outlet for the lower stage' pump normally closed by said piston, and relief valve means normally set for the` pressure of the last pump in series and provided with a control ele-.f

'carried by each pump when said pumps are connected in series, said means comprising a casing and a piston valve in said casing having differential pressure areas, said piston valve being open on one side to the inlet pressure and on the other side Ato the` discharge pressure 'of each particular pump, said casing being provided with a bypass outlet for said particular pump normally closed by-said piston, the respective pressure areas of said piston valve having a ratio substantially identical to the ratio to be maintained between the inlet pressure of the particular pump and the i outlet pressure, and relief valve means normally y set for the pressure of the last pump in series and provided with a control element backed up by the pressure of the remaining pumps when such are operating.

5. A liquid power circuit, comprising in combination, a source of liquid supply, a main pressure line, two continuously operating positive displacement pumps normally drawing liquid from the liquid source, passageways leading from said pumps for conducting liquid under pressure to said main line, bypass valves operatively con- .nected to one of said pumps and adapted to conduct liquid from said one pump to the inlet of the other pump when said bypass valves are open to place said pumps in series, a connection between the main pressure line and said bypass valves to allow opening of said bypass valves to turn the discharge of each of said pumps into the inlet of the next succeeding pump only upon the building up of a predetermined pressure in the main line, means for closing the liquid supply from all but the first one of said series of pumps when said bypass valves are open, differential pressure means responsive to the inlet and outlet pressures of the last pump in series for regulating the proportions of pressure to be carried by each pump when said bypass valves are open, and relief valve means normally set for the pressure of the last pump in series and provided with a control element backed up by the pressure of the rst pump in series, said pumps combining to produce their combined volume when the main line pressure isbeiow said predetermined pressure, and to produce the volume of the last pump in the series when operating in series.

6. A liquid power circuit, comprising in combination, a source of liquid supply, a main pressure line, two continuously operative positive displacement pumps each normally drawing liquid from the liquid source and having progressively decreasing volume capacities, passageways leading from said pumps for conducting liquid under pressure to said main line, a bypass valve in the outlet of the highest capacity pump adapted to admit liquid under pressure from said pump to the /next succeeding pump when said bypass valve is open, connections between the main pressure line and said bypass valve to allow opening. of said bypass valve to place said pumps in series only upon the building up of a predetermined pressure in the main line, means for closing the liquid supply from all of said pumps except the pump iirst in series when said bypass valve is open, means responsive to the inlet and of outlet pressures of each pump in series for regulating the proportions of pressure to be carried by each pump when said bypass valves are open, and relief valve means normally set for the pressure of the last pump in series and provided with a control element backed up by the pressure of the rst pump when such is operating, said pumps combining to produce their combined volume when the main line pressure is below said predetermined pressure, and to produce the volume of the last pump in the series when operating in series.

'7. A liquid power circuit comprising, in combination, a liquid supply, a main pressure line, two continuously operating positive displacement pumps, supply conduits between each of said pumps and the liquid supply, pressure conduits betweeneach of said pumps and the main pressure line, a one-way check valve in one of said last named conduits, a bypass valve in said last named conduit between said check valve and one of said pumps, a by'ipass conduit connecting said bypass valve and the supply conduit of the other of said pumps, a one-Way check valve in said last named supply conduit between said bypass conduit and the liquid supply, a connection between the main pressure line and said bypass valve to allow opening of said bypass valve to turn the discharge of the pump first in series with the supply conduit of the pump second in series only upon the building up of a predetermined pressure in the main line, valve means responsive to the inlet and outlet pressures of each pump for regulating the proportion of mainline pressure to be carried by eachpump when in series, and a pressure balanced relief valve means normally set forthe pressure of the last pump in series and provided with a control element backed up by the pressure of the remaining pump when such is operating, and adapted to open at a lower pressure when said rst named pump fails to operate, said pumps combining to produce their t combined volume when the main line pressure is below said predetermined pressure, and to produce the volume of the last pump in the series when operating in series.

8. In a compound pump circuit, a plurality of upper and lower stage pumps normally connected in parallel to a main pressure line, pressure controlled valve means operative to connect said pumps in series at a certain predetermined pressure in the main line, valve means responsive to the inlet and outlet pressures of each of said pumps but the first in series to predetermine the relative proportions of pressure to be carried by each pump when operating in series, and relief valve means having a control element responsive to pressures in the Ilower stage pumps when such are operating and adapted to open only at a maximum circuit pressure when said pumps operate in series and to open at a lower pressure when a lower stage pump fails to operate.

9. In a compound positive displacement pump circuit, a plurality of positive displacement upper and lower stage pumps connected in series, differential area valve means responsive to pressures in the inlet and outlet of each of said pumps but the rst in series to predetermine the relative proportions of pressure to be carried by each pump when operating in series, and relief valve means having a control element backed by pressures from said lower stage pumps and adapted to open only at a maximum circuit pressure when said pumps all operate in series and to open at a lower pressure when a lower stage pump fails to operate.

10. In a compound pump circuit adapted to produce a total pressure greater than the effective pressure load on any one pump in the circuit when connected in series, a plurality of positive displacement pumps, conduit means for connecting the outlet of each pump but the last in series to the inlet of the next succeeding pump in seriesya main conduit leading from said last pump in series for directing fluid under pressure, means responsive to the inlet and outlet pressures of all but the first pump in series for regulating the proportion of main line pressure to be carried by each pump when in series, and a relief valve means connected to said main conduit normally set for the pressure ofthe last pump in series and provided with a control element backed up by the pressure of the remaining pumps in series when such are operating.

11. In a compound pump circuit, a plurality of to a main pressure line comprising an upper stage pump and one or more lower stage pumps, valve means operable by a predetermined pressure in said main pressure line and adapted, when operated by said predetermined pressure, to shunt the output of said one or more lower stage pumps to the input of the next succeeding pump whereby said pumps are connected in series, the upper stage pump being the last inthe series, a differential valve means connected to the respective outlets of each set of two adjacent pumps for regulating the relative proportions of pressure to be carried by each pump when operating in series, said differential valve means comprising a housing and a differential pressure area piston valve open on one side to the outlet pressure of a specific pump and on the other side to the outlet pressure of an adjacent specific pump, said piston valve normally closing a bypass outlet for said pump inlet, and relief valve means normally set for the pressure of the last pump in series and provided with a control element backed up by the pressure of the remaining pumps in series when such are operating.

12. In a compound positive displacement pump circuit adapted to produce a total pressure greater than the effective pressure load on any one. pump in the circuit when connected in series, a plurality of pumps, conduit means connecting the outlet of each pump but the last in series to the inlet of the next succeeding pump in the series, a main conduit leading from said last pump in series for directing fluid under pressure, and a relief valve connected to said main conduit comprising a housing, a bypass opening in said housing, and a control element in said housing normally blocking said bypass opening and normally set for the pressure of the last pump in series and backed up by the pressure of the remaining pumps in series when such are operating.

13. In a compound positive displacement pump circuit adapted to produce a total pressure greater than the effective pressure load on any one pump in the circuit when connected in series, a plurality of pumps, conduit means connecting the outlet of each pump but the last in series to the inlet of the next succeeding pump in the series, a main conduit leading from said last pump in series for directing uid under pressure, and a relief valve means connected to said main conduit normally set for the pressure of the last pump in series and provided with a control element backed up by the pressure of the remaining pumps in series when such are operating.

14. In a compound positive displacement pump circuit having a liquid supply source, a plurality of positive displacement pumps normally connected to a main pressure line, and adapted to be connected in series, conduit means for connecting the outlet of each pump but the last in series to the inlet of the next succeeding pump, pressure controlled valve means in said conduit means operative at a certain predetermined line pressure to open said conduits and connect the outlet of each of said pumps to the inlet of the next succeeding pump, thereby placing the liquid supply source in series with the main pressure line through each of said pumps in turn, and dierential valve means at the inlet of each of said pumps but the first in series to regulate the relative proportions of the total pressure to be carried by each pump when operating in series, said differential valve means comprising a bypass conduit and a differential area piston for controlling the opening of said bypass conduit,

said piston being open on one side to the inlet 5 of a pump and open at the other side to the outlet of said pump.

15. In a compound positive displacement pump circuit adapted to have pumps in series, each bearing a pressure load lower than the total circuit pressure, a plurality of pumps designed to have progressively decreasing volume capacities when in a series circuit, a fluid supply source and main iiuid pressure line, and means operatively connected to adjacent pumps for regulating the relative proportions of pressure to be carried by each pump when said pumps are connected in series, said means comprising a casing and a. piston valve in said casing having differential pressure areas, said piston valve being open on each side to the respective discharge pressure of said adjacent pumps, said casing being provided with a bypass outlet for the rst in series of said adjacent pumps normally closed by said piston, the respective pressure areas of said piston valve having a ratio substantially identical to the ratio to be maintained between the outlet pressure of the said adjacent pumps.

16. In a compound pump circuit, a plurality of positive displacement pumps to be connected in series, said pumps having progressively decreasing volume capacities in the series circuit, and a pressure control valve operably located between each two adjacent pumps in the series and arranged to control the excess volume of the first in series of the adjacent pumps to maintain a deiinite relationship between the discharge pressures of the two adjacent pumps whereby to insure said pumps will always operate at maximum efliciency.

17. In a compound pump circuit, a plurality of positive displacement pumps to be connected in series, said pumps having progressively decreasing volume capacities in the series circuit, and a differential pressure control valve operably located between each two adjacent pumps in the series and arranged to control the excess volume of the first in series of the adjacent pumps to maintain a deiinite relationship between the discharge pressures of the two adjacent pumps whereby to insure said pumps will always operate at maximum eiciency, said valve comprising a diierential piston and a bypass opening to be controlled thereby, said piston being exposed on each side to the respective discharge pressures of said adjacent pumps whereby excess volume from the first in series of said adjacent pumps will be bypassed to effect the maintenance of proportionatedoads on said pumps. y

18. In a compound pump circuit, a plurality of positive displacement pumps mounted on a common driving shait and to be connected ln series, said pumps having progressively decreasing volume capacities in the series circuit, and a differential pressure control-valve operably located between each two adjacent pumps in the series arranged to control the excess volume of the iirst in series of the adjacent pumps to y,maintain a definite relationship between the discharge pressures of the two adjacent pumps.

HARRY F. VICKERS. 

